Using a cut-in box in a common wall.

I am in the process of adding an interior bedroom to my townhouse. One of the walls is a common wall and fire rated with two sheets of 5/8" drywall. Where I live Romex is code and not conduit. To minimize the cutting into of an existing wall, I want to use cut-in boxes. Is it within code to use these on a fire rated wall or do I have to attach a box directly to a stud?

Interesting question , as I have sensed in recent years that inspectors are very touchy about fire walls.
When dlarivee says the box must be rated....it is true that any pentration in a fire rated door or wall, the device must carry a UL Fire Rating. If it is a fire rated door, the jamb and the door knob hardware must be rated. An entry door which opens to a fire rated hallway, the door, the lockset, and THE VIEWER, if there is one, MUST be UL rated. If you recess a fire extingquisher cabinet into a fire rated wall, a regular steel cabinet is NOT enough. It must be a fire rated cabinet, which will actually have an additional steel liner of minimum thickness. get the picture??

I don't know what it takes for an electrical box to be fire rated. Someone will tell us.

The metallic outlet or switch boxes must be securely fastened to the studs. Openings in the wallboard facing are to be cut so that the clearance between the box and wallboard does not exceed 1/8 inch. The surface area of individual metallic outlet or switch boxes must not exceed 16 square inches. In addition, the entire surface area of the boxes must not exceed
100 square inches per 100 square feet of wall surface.

A minimum horizontal distance of 24 inches must separate metallic boxes located on opposite sides of walls or partitions. This minimum horizontal spacing may be reduced through the use of UL Classified Wall Opening Protective Materials (QCSN), commonly known as “putty pads” or “insert pads.” Further, metallic boxes cannot be installed on opposite sides of walls or partitions in staggered stud constructions unless putty pads or insert pads are installed with the metallic boxes in accordance with the Classification requirements for the protective materials.

With that information, you can determine if what you are proposing is compliant.l